1. Field of the Invention
The present invention relates in general to an apparatus for converting outline data representative of the outline of a visible representation such as letters, symbols and other characters or graphical representations, into dot data representative of image dots to be formed to reproduce the visible representation. More particularly, the present invention is concerned with improvements in the continuity of each stroke of the character or other visible representation.
2. Discussion of the Prior Art
Visible representations or characters such as letters, numerals and symbols represented by character data are printed, displayed or otherwise reproduced according to the character data suitably processed by a computer. Commonly, the character data takes the form of dot data indicating whether a dot should be formed in each of picture elements which are the smallest part of picture image and which determine the resolution of the reproduced image. If a batch of dot data representative of all the characters available for reproduction is prepared and stored in a character data memory, the memory should have an extremely large storage capacity. It is therefore desirable to store a batch of outline data representative of the outlines of the characters, and convert the outline data into the corresponding dot data by suitable data converting means, when the visible representations are printed, displayed or otherwise reproduced, as disclosed in Japanese Patent Publication No. 53-41017.
An example of a data converting apparatus as described above is disclosed in co-pending U.S. patent application Ser. No. 07/425,604 filed Oct. 23, 1989. In the data converting device disclosed therein, the outline of each character represented by outline data is superimposed on a coordinated pixel screen in which picture elements are defined along a plurality of parallel scanning or pixel lines parallel to the X axis of the screen. Each pixel line passes the centers of an array of the picture elements. In converting the outline data into dot data, X-axis coordinate values of intersections between the character outline and each pixel line are calculated. More precisely, the coordinate values of the picture elements which are nearest to the intersections and inside the character outline are calculated. Based on the calculated coordinate values, bits of the dot data are set to indicate the presence of image dots at the positions of the picture elements corresponding to the width of a stroke of the character. These dot data bits are referred to as "dot-forming bits". In this data converting apparatus, each of the pixel lines intersecting one or more strokes of the character has two X-axis coordinate values for each stroke of the character. These two coordinate values define the width of the stroke as measured along the relevant pixel line. Since image dots are formed at and between the two coordinate values, bits of the dot data which correspond to the picture elements at and between the two coordinate values are set to indicate the presence of the image dots. This apparatus is therefore capable of setting at one time the bits of the dot data between the calculated two X-axis coordinates for each stroke, with respect to one pixel line, without determining whether an image dot should be formed at each picture element position within the width of the stroke. Thus, the character outline data may be efficiently and easily converted into the corresponding batch of dot data.
However, there is a possibility that the character outline does not intersect any one of the pixel lines. For example, if a stroke of a character is relatively narrow, a certain portion of the outline of that narrow stroke does not intersect any pixel lines. In this case, no X-axis coordinate values are obtained for that given portion of the stroke, and as a result, the prepared dot data for the stroke do not include dot-forming bits for the given portion of the stroke. Accordingly, the stroke reproduced according to the dot data is discontinuous and has a poor appearance.
The problem of the discontinuity of the stroke indicated above may be eliminated by using a sufficiently large number of scanning lines parallel to the above-indicated pixel lines which pass the centers of the picture elements. For example, the scanning lines are provided at a pitch which is a half of the pitch of the pixel lines. In this case, there is a reduced or no possibility that a stroke of a character does not intersect any one of the scanning lines, whereby each of almost all portions of the stroke has two intersections with the relevant scanning lines. Accordingly, the discontinuity of the stroke reproduced according to the prepared dot data may be reduced or eliminated. However, this arrangement requires an increased time for calculating the coordinates of the intersections between the character outline and the scanning lines, and consequently reduces the above-indicated advantage of efficient conversion of the outline data into the dot data.